Test AWS and GCP authenticated storage access in CI

[bgilbert]

We have kola tests that perform authenticated cloud storage fetches from inside a cloud VM on the same platform. However, we don't run them in upstream CI, and we only run them in downstream CI under limited circumstances. It'd be nice to run these upstream on every PR, especially for #1361, but they're pretty heavyweight: they require building an OS image, uploading it to the cloud, and launching a VM.

One alternative is to construct a test that launches an existing FCOS cloud image, uploads an Ignition binary to it, and manually/hackily invokes the kola external test.